Electrochemical anodic oxidation of a target substrate is in use on various scenes. Such anodic oxidation includes treatment in which a polycrystalline silicon layer is made porous. The outline thereof is such that the target substrate having the polycrystalline silicon layer formed on the surface thereof is electrically connected to a positive potential pole of a power supply via a conductor and immersed in a hydrofluoric acid solution dissolved in a solvent (for example, ethyl alcohol). An electrode made of, for example, platinum is immersed in the hydrofluoric acid solution, in other words, in a chemical, and is electrically connected to a negative potential pole of the above-mentioned power supply. Further, the polycrystalline silicon layer on the target substrate immersed in the chemical is irradiated with light by a lamp.
This causes the polycrystalline silicon layer to partly melt in the hydrofluoric acid solution. Pores are formed where the polycrystalline silicon layer has been melted, so that the silicon layer is turned into a porous structure. The photoirradiation by the lamp is intended for producing holes necessary for the reaction of the above-mentioned melting and pore formation in the polycrystalline silicon layer. For reference, such reaction in the polycrystalline silicon layer in the anodic oxidation is explained, for example, as follows.Si+2HF+(2−n)e+→SiF2+2H++ne−SiF2+2HF→SiF4+H2 SiF4+2HF→H2SiF6 Here, e+ is a hole and e− is an electron. Therefore, this reaction requires holes as a precondition and is different from simple electrolytic polishing.
The porous silicon thus produced is made suitable as a highly efficient field emission electron source by further forming a silicon oxide layer on a micro-level surface thereof, which is disclosed in, for example, Japanese Patent Laid-open No. 2000-164115, Japanese Patent Laid-open No. 2000-100316, and so on. The use of such a porous silicon as the field emission electron source has been drawing attention as opening a door to realizing a new flat display device.
In the anodic oxidation requiring the photoirradiation as described above, a cathode electrode is naturally positioned between the light emitting lamp and a treatment part of the target substrate. First, the cathode electrode needs to satisfy such a condition that it faces the entire surface of the treatment part thoroughly in terms of position so as to cause the treatment part to act uniformly. Meanwhile, the cathode electrode needs to allow the light emitted by the lamp to pass therethrough and to reach the treatment part.
For these purposes, for example, an electrode in a grid form having the same spread as a planar spread of the treatment part is used. Consequently, the cathode electrode can face the entire surface of the treatment part thoroughly in terms of position and allows the light to pass through opening portions in the grid so that the light reaches the treatment part.
Even when the electrode is made in the grid form, however, the shadow of the electrode in the grid form appears on the treatment part. Consequently, further minute observation would show that photoirradiation amounts on the treatment part are not uniform in the surface thereof, which poses some limit to uniformity of treatment as the anodic oxidation.